Lithographic apparatus, excimer laser and device manufacturing method

ABSTRACT

A CD-pitch dependency for a lithographic pattern printing process is related to the spectral intensity distribution of radiation used for projecting the pattern. A CD-pitch dependency can vary from one system to another. This can result in an iso-dense bias mismatch between systems. The invention addresses this problem by providing a lithographic apparatus including an illumination system for providing a projection beam of radiation, a projection system for projecting a patterned beam onto a target portion of a substrate, and a substrate table for holding the substrate, with a controller to provide an adjustment of the spectral distribution of radiant intensity of the projection beam. The adjustment of the spectral intensity distribution is based on data relating to an iso dense bias, and includes a broadening of the spectral bandwidth or a change of shape of the spectral intensity distribution.

RELATED APPLICATIONS

This is a continuation in part of U.S. patent application Ser. No.11/019,531, filed Dec. 23, 2004 which is now U.S. Pat. No. 7,534,552,hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus, an excimerlaser and a device manufacturing method. This invention also relates toa device manufactured thereby.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, which is alternatively referredto as a mask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC, and this pattern can beimaged onto a target portion (e.g., comprising one or several dies) on asubstrate (e.g., a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Known lithographic apparatus include so-called steppers, inwhich each target portion is irradiated by exposing an entire patternonto the target portion at once, and so-called scanners, in which eachtarget portion is irradiated by scanning the pattern through theprojection beam in a given direction (the “scanning”-direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection.

Between the reticle and the substrate is disposed a projection systemfor imaging the irradiated portion of the reticle onto the targetportion of the substrate. The projection system includes components fordirecting, shaping or controlling the projection beam of radiation. Theprojection system may, for example, be a refractive optical system, or areflective optical system, or a catadioptric optical system,respectively including refractive optical elements, reflective opticalelements, and both refractive and reflective optical elements.

Generally, the projection system comprises a device to set the numericalaperture (commonly referred to as the “NA”) of the projection system.For example, an adjustable NA-diaphragm is provided in a pupil of theprojection system.

An illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens”. The illumination system of theapparatus typically comprises adjustable optical elements for setting anouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of an intensity distribution upstream of themask, in a pupil of the illumination system. A specific setting ofσ-outer and σ-inner may be referred to hereinafter as an annularillumination mode. Controlling the spatial intensity distribution at apupil plane of the illumination system can be done to improve theprocessing parameters when an image of the illuminated object isprojected onto a substrate.

Microchip fabrication involves the control of tolerances of a space or awidth between devices and interconnecting lines, or between features,and/or between elements of a feature such as, for example, two edges ofa feature. In particular the control of space tolerance of the smallestof such spaces permitted in the fabrication of the device or IC layer isof importance. Said smallest space and/or smallest width is referred toas the critical dimension (“CD”).

With conventional projection lithographic techniques it is well knownthat an occurrence of a variance in CD for both semi-dense and isolatedfeatures may limit the process latitude (i.e., the available depth offocus in combination with the allowed amount of residual error in thedose of exposure of irradiated target portions for a given tolerance onCD). This problem arises because features on the mask having the samenominal critical dimensions will print differently depending on theamount of defocus (out of a plane of best focus) of the part of thetarget portion where the feature is imaged, due to, for example,substrate topography, image curvature or substrate unflatness.

A difference in printed CD between two similar features such as contactholes at two respective, different locations on a substrate havingcorresponding different focal positions will be referred to hereinafteras a CD-focus error. For example, a contact hole having a particularcontact hole size and disposed at a first position in the pattern, willprint differently from the same feature having the same size anddisposed at a second position, when at respective conjugate first andsecond positions at substrate level the respective exposed substrateareas are disposed at respective different focus positions. Hence, whenboth contact holes are to be printed simultaneously, a positiondependent variation of printed CD is observed. Data describing aspecific CD-focus dependency are generally represented by a plot of CDversus focus for a constant exposure dose, and referred to as a Bossungcurve. The phenomenon CD-focus error is a particular problem inphotolithographic techniques.

Conventional lithographic apparatus do not directly address the problemof CD-focus error. Conventionally, it is the responsibility of the usersof conventional lithographic apparatus to attempt to maintain CD-focuserror within tolerance by compensating for a defocus (for example byadjusting or varying the focus position of the substrate during anexposure), or by optimizing the setting of apparatus optical parameters,such as the NA of the projection lens or the σ-outer and σ-innersettings, or by designing the mask in such a way that focus dependencyof dimensions of printed isolated and semi-dense features is reduced.However, such a measure to reduce CD-focus error may lead to an increaseof other lithographic prosess errors or sensitivities, and may thereforestill adversely affect the process latitude.

Given a pattern to be provided by a patterning device, and to be printedusing a specific lithographic projection apparatus including a specificradiation source, one can identify data relating to CD-focus error whichare characteristic for that process, when executed on that lithographicsystem. In a situation where different lithographic projection apparatus(of the same type and/or of different types) are to be used for the samelithographic manufacturing process step, there is a problem of mutuallymatching the corresponding different CD-focus dependencies, such as toreduce CD variations occurring in the manufacturing process.

An actual CD-focus dependency as described above may be varying in time.For example, due to lens heating the aberration of the projection systemmay vary, and or due to heating and other instabilities properties suchas illumination settings, and exposure dose of radiation energy may varyin time. Therefore there is the problem of controlling and keepingwithin tolerance a desired CD-focus dependency.

A lithographic process is generally characterized by a process latitudeor process window such as, for example, an Exposure-Defocus window, alsoreferred to as an ED-window. An ED-window indicates the available depthof focus in combination with the allowed amount of residual error in theexposure dose of irradiated target portions for a given tolerance on CD.

With conventional projection lithographic techniques it is well knownthat an occurrence of a variance in CD due to focus variations andexposure dose variations may limit the process latitude. In general theuseable focus-range of an ED-window is asymmetric around a focusposition referred to as the Best Focus or BF. At best focus, a change ofCD as a function of a change of focus position of the substrate issmallest or even zero (in the latter case the Bossung curve is locally“iso-focal”, i.e., parallel to the focus axis of the Bossung plot).Typically the available range of focus for contact holes tends to belocated asymmetrically around the position BF of best focus, and theCD-focus dependency is then asymmetric with respect to best focus. Thiscorresponds to a process whereby printed contact holes close earlier inone defocus direction, as compared to the other defocus direction. Acorresponding Bossung curve is typically shaped as a tilted and curvedline segment. An asymmetric CD-focus dependency is limiting thelithographic process latitude, in particular for processes applied towafers having a considerable topography.

SUMMARY OF THE INVENTION

The present inventors have identified the following. Techniques areknown to enhance the depth of focus for a projection lithographicprocess by manipulating the spectral distribution of radiant intensityof the projection beam. Generally, radiation used for exposure isprovided by an excimer laser; for example, a KrF excimer laser operatingat 248 nm wavelength or an ArF excimer laser operating at 193 nmwavelength may be used. The spectral distribution of radiant intensityprovided by such lasers comprises a spectral intensity peak having asymmetric shape with respect to a peak wavelength λ_(p). The bandwidthof the spectral peak may be expressed as a full-width half-maximumbandwidth (referred to as FWHM bandwidth) or alternatively as thebandwidth within which 95% of the total output power of the laser iscontained (referred to as the E95 bandwidth), with the peak wavelengthλ_(p) typically centered within said bandwidths.

The finite magnitude of the bandwidth introduces a “smear out” of theimage of a feature over a focus range around a best focus position BF.Said smear out is represented by a plurality of images displaced alongthe optical axis of the projection system, in accordance with aplurality of radiation wavelengths (in a range of wavelengths centeredat λ_(p)). The plurality of axially displaced images is formed by theprojection system due to the presence of residual axial chromaticaberration of the projection system. If F is the distance between theplane of best focus corresponding to the radiation wavelength λ_(p) andan image plane corresponding to the radiation wavelength λ, the effectof axial chromatic aberration is described by dF/dλ=AC, where AC is aconstant. Therefore, to a good approximation the effect of a constantdefocus of the substrate over a distance F, during exposure, is the sameas the effect of a change of wavelength Δλ given by Δλ=F/AC and exposingwith radiation of this changed wavelength with the substrate held in thebest-focus focal plane.

The effects of finite spectral bandwidth of the laser radiation can bemodeled by linearly converting a symmetric laser spectral distributionof exposure intensity into a symmetric focus distribution using the lensproperty AC defined by dF/dλ=AC. Over a fairly wide range of wavelengthsthe laser spectrum can be converted linearly into a focus spectrum usingthis lens dependency dF/dλ (see FIG. 1a U.S. Patent ApplicationPublication No. 2002/0048288 A1).

A finite laser bandwidth results in the re-distribution of the aerialimage through focus. The total aerial image will be a sum of the aerialimages, each aerial image defocused in accordance with F=AC Δλ, andweighted by the relative exposure intensity at the wavelengthλ=λ_(p)+Δλ.

This addition of (generally defocused) images has an effect on the imagecontrast at wafer level. A relatively large laser bandwidth introduces arelatively low image contrast at wafer level, but at the same time theavailable depth of focus is increased. However, above mentioned problemof contact holes closing earlier in one defocus direction, as comparedto the other defocus direction, limiting the lithographic processlatitude, remains.

It is an object of the present invention to obviate or mitigate one ormore of the aforementioned problems in the prior art. In particular, itis an object of the invention to provide improved control over CD-focusdependency.

The present inventors have identified that providing an asymmetricspectral intensity distribution of the laser radiation can be used tocompensate an asymmetry of imaging parameters (such as printed CD) withrespect to focus and/or best focus, and in particular to reduce theasymmetry of an asymmetric CD-focus dependency.

According to an aspect of the invention there is provided a lithographicapparatus comprising a radiation system for providing a beam ofelectromagnetic radiation having a spectral distribution of radiantintensity, a support structure for supporting a patterning device, thepatterning device serving to impart the beam of radiation with a patternin its cross-section, a substrate table for holding a substrate, aprojection system for projecting the beam of radiation after it has beenpatterned onto a target portion of the substrate, and a controllerconfigured and arranged to provide an adjustment of said spectraldistribution of radiant intensity based on data relating to exposure ata first focal position and exposure at a second focal position andrepresenting a corresponding first printed size and second printed sizeof the feature.

The present invention provides an advantage of adjustment of thespectral distribution of radiant intensity I(λ): using data relating toexposure at a first focal position and exposure at a second focalposition, it is possible to either match system to system focus behavioror to enhance the symmetry of focus behaviour with respect to a positionof best focus.

According to a further aspect of the invention there is provided anexcimer laser having a bandwidth-controller arranged to control abandwidth of the spectral distribution of radiant intensity, and wherebythe bandwidth-controller is constructed and arranged to spectrally shiftthe bandwidth in reaction to a user supplied signal representative for aselected bandwidth shift of the spectral distribution.

According to an aspect of the invention there is provided a devicemanufacturing method comprising providing a beam of electromagneticradiation having a spectral distribution of radiant intensity,patterning the beam of radiation with a pattern in its cross-sectionusing a patterning device, projecting the patterned beam of radiationonto a target portion of a substrate, and adjusting of said spectraldistribution of radiant intensity in accordance with data relating toexposure at a first focal position and exposure at a second focalposition and representing a corresponding first printed size and secondprinted size of the feature.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion,” respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g., having a wavelength in the range of5-20 nm).

The term “patterning device” used herein should be broadly interpretedas referring to devices that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

Patterning devices may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned. The support structure supports, i.e., bearsthe weight of, the patterning device. It holds the patterning device ina way depending on the orientation of the patterning device, the designof the lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support can be using mechanical clamping, vacuum, or other clampingtechniques, for example electrostatic clamping under vacuum conditions.The support structure may be a frame or a table, for example, which maybe fixed or movable as required and which may ensure that the patterningdevice is at a desired position, for example with respect to theprojection system. Any use of the terms “reticle” or “mask” herein maybe considered synonymous with the more general term “patterning device.”

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system.”

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.,water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion techniques are well known in the artfor increasing the numerical aperture of projection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic apparatus according to a furtherembodiment of the invention;

FIG. 3( a) illustrates an example of an asymmetric spectral intensitydistribution with a location of a peak wavelength, a center wavelength,and E95 wavelengths;

FIG. 3( b) illustrates examples of symmetric spectral intensitydistributions;

FIG. 3( c) examples of asymmetric spectral intensity distributions;

FIG. 4( a) is an example of a lithographic printing processcharacterized by an asymmetric Bossung curve with indication of auseable focus range;

FIG. 4( b) is an example of a symmetric Bossung curve obtained with themethod according to the invention, by using a radiation beam with anasymmetric spectral intensity distribution in the process represented byFIG. 4( a);

FIG. 5 illustrates a symmetric spectral intensity distribution as asuperposition of two spectrally overlapping intensity distributions andan asymmetric spectral intensity distribution as a superposition of twomutually displaced spectral intensity distributions;

FIG. 6 shows a schematic representation of Bossung curves for dense andisolated features;

FIG. 7 illustrates a symmetric spectral intensity distribution and aweight factor representing the intensity distribution;

FIG. 8 shows an example of Bossung curves for dense and isolatedfeatures, and the effect of an increase of spectral bandwidth;

FIG. 9 illustrates an asymmetric laser spectral intensity distributionand an representative weight function consisting of two adjacent blockshaped sections;

FIG. 10 shows an example of Bossung curves for dense and isolatedfeatures, and the effect of an increase of asymmetry of a spectralintensity distribution;

FIG. 11 schematically depicts the effect of a transition from arelatively narrow, symmetric spectral intensity distribution to abroader symmetric and to a broader asymmetric spectral intensitydistribution on a Bossung curve for an isolated feature;

FIG. 12 depicts a simulated effect of symmetric increase of FWHM (FullWidth Half Maximum) for nominal 65 nm isolated lines;

FIG. 13 depicts symmetric laser spectral intensity distributionsdistribution as used for the simulations of FIG. 12;

FIG. 14 illustrates four intensity distributions for use with asimulation of the effect of increased intensity distribution asymmetryat a constant FWHM (Full Width Half Maximum);

FIG. 15 shows the Bossung curves for dense and isolated featurescorresponding to the spectra of FIG. 14;

FIG. 16 depicts Bossung curves for dense and isolated featurescorresponding to the spectra of FIG. 14 after correction for focusshift;

FIG. 17 is a schematic representation of introduction of a wafer tiltdue to rotation around an x-axis;

FIG. 18 is a schematic representation illustrating an analogy of effectof wafer tilt on focus history seen by a part of the wafer during scanand a normal exposure in the absense of tilt but now with a laserspectral intensity distribution having a finite bandwidth.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL for providing a        projection beam PB of radiation (e.g., UV radiation or EUV        radiation).    -   a first support structure (e.g., a mask table) MT for supporting        a patterning device (e.g., a mask) MA and connected to first        positioning actuator PM for accurately positioning the        patterning device with respect to item PL;    -   a substrate table (e.g., a wafer table) WT for holding a        substrate (e.g., a resist-coated wafer) W and connected to        second positioning actuator PW for accurately positioning the        substrate with respect to item PL; and    -   a projection system (e.g., a refractive projection lens) PL for        imaging a pattern imparted to the projection beam PB by        patterning device MA onto a target portion C (e.g., comprising        one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjustable optical elements AM foradjusting the angular intensity distribution of the beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asπ-outer and π-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL generally comprises various other components, such as anintegrator IN and a condenser CO. The illuminator provides a conditionedbeam of radiation, referred to as the projection beam PB, having adesired uniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on themask table MT. Having traversed the mask MA, the projection beam PBpasses through the lens PL, which focuses the beam onto a target portionC of the substrate W. With the aid of the second positioning actuator PWand position sensor IF (e.g., an interferometric device), the substratetable WT can be moved accurately, e.g., so as to position differenttarget portions C in the path of the beam PB. Similarly, the firstpositioning actuator PM and another position sensor (which is notexplicitly depicted in FIG. 1) can be used to accurately position themask MA with respect to the path of the beam PB, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning actuator PM and PW.However, in the case of a stepper (as opposed to a scanner) the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following example modes:

In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C at once (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

In scan mode, the mask table MT and the substrate table WT are scannedsynchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning devices, such as a programmable mirror array ofa type as referred to above.

A controller CON is provided to provide an adjustment of the spectraldistribution of radiant intensity of the beam PB based on data relatingto exposure at a first focal position and exposure at a second focalposition and representing a corresponding first printed size and secondprinted size of the feature.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus of FIG. 2, in contrast to theapparatus in FIG. 1, is of a reflective type (e.g., employing areflective mask).

The apparatus of FIG. 2 comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g., UV radiation or EUV radiation);    -   a support structure (e.g., a mask table) MT constructed to        support a patterning device (e.g., a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a projection system (e.g., a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.,        comprising one or more dies) of the substrate W.

A difference between the spectral bandwidth of lasers which are part ofrespective lithographic projection apparatus result in differencesbetween a CD-focus dependency of printed CD for these respectiveapparatus. Further, an asymmetric CD-focus dependency with respect tobest focus may result in a too small process latitude. The presentinvention seeks to address these problems by providing an apparatuswhich is equipped with a controller CON configured and arranged toprovide an adjustment of the symmetry of the spectral distribution ofthe laser radiation whereby the adjustment is aimed at affecting theCD-focus dependency of the lithographic apparatus. The adjustment may bea dynamic adjustment to compensate for variations in time of a CD-focuserror. Such variations in time may, for example, be caused by lensheating due to absorption of laser beam radiation during exposure. TheCD-focus dependency is specific for the apparatus in combination withthe layout of the mask pattern and other process parameters andproperties such as for example the illumination mode and setting, theexposure time, the resist type, the specific lens aberrations, as wellas settings for the pre-exposure and post exposure processing steps.

An asymmetric CD-focus dependency can be affected, according to thepresent invention, by adjusting the shape of the spectral intensitydistribution of the laser beam, and in particular, by introducing anasymmetry into the shape of the spectral intensity distribution. Anexcimer laser generally is provided with means to control and adjust thespectral distribution of the emitted laser radiation. For example U.S.Patent Application Publication No. 2002/0048288A1 relates to an excimerlaser provided with a controller of a line-narrowing device forcontrolling a the spectral distribution of the laser beam. Thecontroller is arranged to adjust the bandwidth of the spectraldistribution by dithering a wavelength tuning mirror in phase with therepetition rate of the laser. The line narrowing unit comprises agrating and a fast tuning mechanism, and the controller controls amonitoring of the laser beam to determine bandwidth of individual pulseslaser pulses, and a periodically adjusting of the tuning mechanismduring a series of pulses so that the wavelengths of some pulses in theseries of pulses are slightly longer than a target wavelength and thewavelengths of some pulses in the series of pulses are slightly shorterthan the target wavelength in order to produce for the series of pulsesan effective laser beam spectrum having at least two spectral peaks. Inthe latter case, the spectral distribution of radiant intensity may forexample be a superposition of a first and a second peaked spectralintensity distribution having a respective equal first and secondfull-width half-maximum bandwidth, and a respective equal first andsecond intensity. The spectral peaks feature a respective first andsecond peak wavelength, and the difference Δλ_(p) between the first andsecond peak wavelength is adjustable.

Similarly, U.S. Pat. No. 5,303,002 relates to an excimer laser whichgenerates a beam of radiation whereby the spectral distribution ofradiant intensity of the laser beam of radiation comprises a pluralityof narrow spectral bands of radiation. A line narrowing device isarranged to select one or more line narrowed outputs to be used for thelithographic process. Each of the outputs may have an attenuator whichcan adjust the intensity of each spectral band independently so that anasymmetric spectral intensity distribution can be provided. Thecorresponding radiation beams pass through a gain generator and arecombined to produce a beam of radiation with the desired spectraldistribution. The spectral intensity distribution may be an asymmetricdistribution, i.e. a distribution with a spectral shape deviating from asymmetric shape with respect to a center wavelength λ_(c).

As mentioned above, generally an ED-window is not symmetric around bestfocus (e.g., contact holes generally close earlier in one defocusposition than the other). Also, a Bossung curve may be a curved, tiltedline segment. In the present invention a spectral intensity distributionwhich is asymmetric with respect to a center wavelength λ_(c) is used tocorrect for such a tilt of a Bossung curve, and to make the UseableFocus Range (viable process/production window) substantially symmetricaround best focus BF. Thus the CD-focus dependency of features is madesymmetric with respect to best focus using a projection beam having anasymmetric spectral intensity distribution.

Referring to FIG. 3( a), there is shown an example of an asymmetricspectral distribution 300. The wavelengths λ₁ and λ₂ in FIG. 3( a)define the E95 bandwidth represented by the arrow 301. The wavelengthλ_(c) is the center wave length, i.e. the wavelength at the center ofthe range [λ₁, λ₂]. The curve 300 represents the spectral intensitydistribution I(λ), which is peaked at a peak wavelength λ_(p). Ingeneral, an asymmetric intensity distribution is characterized by theinequality I(λ−λ_(c))≠I(λ_(c)−λ). A measure for asymmetry may beexpressed in terms of the moments of intensity MI_(left) and MI_(right)defined as

$\begin{matrix}{{{MI}_{left} = {{\int_{\lambda_{1}}^{\lambda_{c}}{\lambda \times {I(\lambda)}\ {\mathbb{d}\lambda}\mspace{14mu}{and}\mspace{14mu}{MI}_{right}}} = {\int_{\lambda_{c}}^{\lambda_{2}}{\lambda \times {I(\lambda)}\ {\mathbb{d}\lambda}}}}},} & (1)\end{matrix}$

and the spectrum may be referred to as asymmetric when MI_(left) isdifferent from MI_(right). For example, the spectrum may be referred toas asymmetric when the spectral intensity distribution I(λ) is anasymmetric distribution whereby the moments of intensity, as defined inequation (1), satisfy the inequality

${1.05 \leq \frac{{MI}_{left}}{{MI}_{right}}},{or}$$0.95 \geq {\frac{{MI}_{left}}{{MI}_{right}}.}$

FIG. 3( b) shows an example of symmetric spectral radiationdistributions 302,304 and 306.

FIG. 3( c) shows an example of asymmetric spectral radiationdistributions 303, 305 and 307.

Referring now to FIG. 4( a), there is shown an example of an asymmetricBossung curve with indication of the useable focus range. The Bossungcurve corresponds to a lithographic process which in the presense of asymmetric spectral intensity distribution has an asymmetric CD-focusdependency. Note that in negative defocus, the process window is notlimited by the indicated lower process limit. by e.g., pattern collapse.In positive defocus the CD is limited by the preselected range ofallowable CD values.

Referring to FIG. 4( b), there is shown an example of a symmetricBossung curve obtained after applying an asymmetric laser intensitydistribution to the lithographic process represented by the Bossungcurve of FIG. 4( a). Again, an indication of the useable process windowis given. Note that in negative defocus, the process window is nowlimited by the preselected range of allowable CD values.

According to an aspect of the invention the source SO in FIG. 1 is anexcimer laser providing a pulsed beam of laser radiation. The lasercomprises bandwidth monitoring equipment and wavelength tuning equipmentpermitting bandwidth control of the laser beam by a bandwidth-controllerof the laser. The bandwidth-controller of the laser is generally used tomaintain a preselected bandwidth (compensating, for example, changes inthe laser-gain medium over the life of the laser), in accordance with aselection made by the laser manufacturer. According to the presentinvention, however, the bandwidth-controller of the laser is providedwith an input channel arranged for receiving a signal representative fora selected asymmetry of the spectral distribution in accordance with aselection made by the user of the laser. For example, the signal can beprovided by the controller CON of the lithographic apparatus accordingto the present invention. With an eximer laser featuring auser-selectable spectral bandwidth asymmetry the adjustmet of CD-fucusdependency according to the present invention can be provideddynamically, for example, during a sanning exposure of a target portionC or during a plurality of exposures of target portions C covering asubstrate. Both intra-die and inter die controll of CD-focus dependencyis obtained this way. Similarly, an eximer laser provided withuser-selectable spectral asymmetry setting can be used to obtainCD-focus dependency matching between different apparatus, in accordancewith the present invention.

FIG. 5 illustrates a spectral distribution of radiant intensity 500 as asuperposition of a first peaked spectral intensity distribution 501 anda second peaked spectral intensity distribution 502 having a respectivefirst bandwidth 503 and second bandwidth 504. The first and second peakwavelengths λ_(p1) and λ_(p2) are equal, but the second peak intensityis lower than the first peak intensity. FIG. 5 further illustrates theeffect of providing, through control of the line width narrowing deviceof the pulsed excimer laser, an adjustment comprising a change Δλ_(p) ofthe difference between the first and second peak wavelength. Theadjustment is (the difference λ_(p1)−λ_(p2) in FIG. 5 being initiallyzero) in the present example equal to the difference λ_(p2)−λ_(p1). Theresulting intensity distribution 506 has an asymmtery with respect to acenter wavelength λ_(c).

Referring to FIG. 6 there is shown a schematic representation of aBossung curve 600 typical for an isolated feature and a Bossung curve601 typical for the feature in dense arrangement, i.e., arranged at aduty cycle 1:1. The Bossung curve 600 represents a plot of printedcritical dimension for the feature in isolated arrangement, and thecorresponding CD is denoted by CD_(iso), as it would be obtained withexposure in different focal positions. The exposure energy is a constantalong the plots 600 and 601. The different focal positions are given bythe focal coordinate F (above referred to as a “defocus”), which definesthe position of the substrate with respect to a position of best focusBF.

Typically, the printed critical dimension CD_(dense) of the densefeature does not depend (to a first approximation) on focal position,because of the extended depth of focus resulting from two beam imaging.Generally, imaging of dense features is arranged such that only twodiffracted orders of radiation, as emerging from the pattern, arecaptured by the imaging projection lens.

The printed critical dimension CD_(iso) may be modelled as a polynomialof F according toCD _(iso) =A ₀ +A ₁ F+A ₂ F ² +A ₄ F ⁴,  (2)

whereby the coefficient A₀ represents the printed CD at best focus.Further, the coordinate F may be expressed in terms of an absolute focuscoordinate f defined by

F=f−f_(BF), where the coordinate f_(BF) is the absolute coordinate,along the z-axis, of the best focus position BF.

In the absence of a so-called linear focus term, i.e. when A₁=0, theresulting second order approximation denoted by CD_(iso) (0,2; f) ofCD_(iso) is then given byCD _(iso)(0,2;f)=A ₀ +A ₂(f−f _(BF))².  (3)

In contrast, the Bossung curve for the dense feature may simply bemodeled as CD_(dense)=B₀. Thus, at best focus BF, the dense features areprinted at a width B₀, and the isolated features at a width A₀, and theiso-dense bias between thee features would be A₀-B₀ nm.

In accordance with the present invention, the effects of finite spectralbandwidth on the Bossung curve can be modeled by linearly converting asymmetric spectral intensity distribution of the laser beam into asymmetric focus distribution using the lens property AC defined bydF/dλ=AC. Since F=f−f_(BF), also df/dλ=AC at or near best focusposition. The laser bandwidth results in the re-distribution of theaerial image through focus. The total aerial image will be a sum of theaerial images, each aerial image defocused in accordance with F=AC Δλ,and weighted by the relative exposure intensity at each wavelength λ.The weighting may be expressed by a weight-function W in accordance withthe spectral distribution of radiant intensity I(λ) of the laserradiation.

The resulting printed CD incorporating the effect of the addition of the(generally defocused) images may be represented by CD_(av), and can beapproximated by the following averaging:

$\begin{matrix}{{{{CD}_{av}(f)} = \frac{\int_{f - {\frac{1}{2}F_{BW}}}^{f + {\frac{1}{2}F_{BW}}}{{{CD}\ }_{iso}\left( f^{\prime} \right){W\left( {f^{\prime} - f} \right)}{\mathbb{d}f^{\prime}}}}{\int_{f - {\frac{1}{2}F_{BW}}}^{f + {\frac{1}{2}F_{BW}}}{{W\left( {f^{\prime} - f} \right)}{\mathbb{d}f^{\prime}}}}},} & (4)\end{matrix}$

where the “bandwidth” F_(BW) represents the focus range equivalent tothe bandwidth of the spectral intensity distribution. For example, withλ₁ and λ₂ being the E95 bandwidth wavelengths, F_(BW) can be defined asF_(BW)=AC(λ₁−λ₂). The weight function W(f) is proportional to thespectral distribution of radiant intensity I(λ) and can be obtained fromI(λ) by expressing I(λ) as a function of (λ−λ_(c)), and writing λ−λ_(c)as an equivalent focal coordinate f with (λ−λ_(c))=f/AC, in view of thelens property df/dλ=AC.

For simplicity it will be assumed that the weight function W(f) inaccordance with the symmetric intensity distribution 302 of FIG. 3 canbe approached by a block function 700, as illustrated in FIG. 7.

Combination of this approximation with the approximation CD_(iso) (0,2;f) for the printed CD of an isolated feature, results in the followingprediction for the average CD (at best focus) of a feature due to theintroduction of a finite laser bandwidth (resulting in there-distribution of the aerial image over a focus range of from −½F_(BW)to ½F_(BW)):

$\begin{matrix}\begin{matrix}{{{CD}_{av}\left( f_{BF} \right)} = \frac{\int_{f_{BF} - {\frac{1}{2}F_{BW}}}^{f_{BF} + {\frac{1}{2}F_{BW}}}{{{CD}_{iso}\left( {0,{2;f^{\prime}}} \right)}{\mathbb{d}f^{\prime}}}}{\int_{f_{BF} - {\frac{1}{2}F_{BW}}}^{f_{BF} + {\frac{1}{2}F_{BW}}}{\mathbb{d}f^{\prime}}}} \\{= {A_{0} + {\frac{1}{12}A_{2}F_{BW}^{2}}}}\end{matrix} & (5)\end{matrix}$

From the above equation it is clear that the change ΔCD_(iso) in printedcritical dimension at best focus (due to a change from idealmonochromatic radiation to the introduction of a certain laser bandwidthresulting in a through focus re-distribution of the image over a focusrange from −½F_(BW) to ½F_(BW)) is given by

${\Delta\;{CD}_{iso}} = {{{A_{2} \cdot \frac{1}{12}}F_{BW}^{2}} \sim F_{BW}^{2}}$

In contrast, no such change occurs for the size of the dense features,since in the present approximation CD_(dense) is a constant value,independent of focus position: CD_(dense)=B₀, in accordance with theiso-dense characteristics as illustrated in FIG. 4.

FIG. 8 schematically illustrates the effect of the change from an idealpractically monochromatic radiation spectrum of the laser beam to theintroduction of a finite laser bandwidth in accordance with the presentapproximation. The arrow 800 represents the (focus independent) shiftΔCD_(iso) of the Bossung curve 600 representing the printed CD asobtained with the exposure process using a practically monochromatic(not bandwidth broadened) laser radiation spectrum, and the curve 810 isthe Bossung curve for the increased laser bandwidth. Since generally theBossung curve for the feature in dense arrangement is less or notsensitive to a change of spectral bandwidth, the adjustment of spectralbandwidth can be used for adjusting the CD-pitch dependency.

Assuming that the energy dependence of the CD is focus independent anundesired residual impact of laser bandwidth on printed CD could beeasily compensated in order to maintain the CD of a reference feature(such as for example the dense lines in the present embodiment)unaltered.

The same approximation as described above can be generalized for anarbitrary defocus position F (and using F=f−f_(BF)) as follows:

$\begin{matrix}{\begin{matrix}{{{CD}_{av}(F)} = \frac{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF} + {\frac{1}{2}F_{BW}}}{{{CD}_{iso}\left( {0,{2;f^{\prime}}} \right)}{\mathbb{d}f^{\prime}}}}{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF} + {\frac{1}{2}F_{BW}}}{\mathbb{d}f^{\prime}}}} \\{= {A_{0} + {A_{2}F^{2}} + {\frac{1}{12}A_{2}F_{BW}^{2}}}}\end{matrix},} & (6)\end{matrix}$

The change in CD induced by the re-distribution of the aerial image overa focus range from −½F_(BW) to ½F_(BW) is independent of the focusposition F and is proportional with F_(BW) ².

For the fourth order focus term in Equation (2) can be derived thefollowing contribution CDav(4) to CDav:

$\begin{matrix}{\begin{matrix}{{{CD}_{av}\left( {4,F} \right)} = \frac{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF} + {\frac{1}{2}F_{BW}}}{{A_{4}\left( {f^{\prime} - f_{BF}} \right)}^{4}{\mathbb{d}f^{\prime}}}}{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF} + {\frac{1}{2}F_{BW}}}{\mathbb{d}f^{\prime}}}} \\{= {A_{4}\left( {F^{4} + {\frac{1}{2}F^{2}F_{BW}^{2}} + {\frac{1}{80}F_{BW}^{4}}} \right)}}\end{matrix}.} & (7)\end{matrix}$

Equation 7 shows that there is now a defocus-dependent shift as well asa constant shift of the Bossung curve.

Similarly, for a first order focus term in Equation (2) can be derivedthe contribution CDav(1) to CDav:

$\begin{matrix}\begin{matrix}{{{CD}_{av}\left( {1,F} \right)} = \frac{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF} + {\frac{1}{2}F_{BW}}}{{A_{1}\left( {f^{\prime} - f_{B}} \right)}{\mathbb{d}f^{\prime}}}}{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF} + {\frac{1}{2}F_{BW}}}{\mathbb{d}f^{\prime}}}} \\{= {A_{1}F}}\end{matrix} & (8)\end{matrix}$

So the re-distribution of the aerial image over a focus range from−½F_(BW) to ½F_(BW) does not impact the linear focus term.

According to an embodiment of the invention, the spectral distributionof radiant intensity comprises a spectral intensity peak having, withrespect to a center wavelength, a symmetric shape and wherein saidadjustment comprises a change of the symmetric shape into an asymmetricshape with respect to the center wavelength.

An asymmetric spectral distribution of radiant intensity of the laserbeam can be provided, for example, by differently attenuating each of aplurality of narrow spectral bands of radiation in a line narrowingdevice which is arranged to select a plurality of line narrowed outputsto be used for the lithographic process. In FIG. 9 a asymmetricintensity distribution I(λ) is represented by the plot 300. Similar tothe embodiment described above, the intensity distribution may beapproximated by adjacent, block shaped intensity distributions. Inparticular, as is illustrated in FIG. 9, in the present embodiment theintensity distribution is modelled as two adjacent block functions 910and 920, of equal area, and different width. The E95 wavelengths λ₁ andλ₂ define a total bandwidth equivalent to the focus range 901 with amagnitude denoted by

$\frac{3}{2}F_{{BW},}$and the spectrum is approximated by the left block function 910 of width

$\frac{1}{2}F_{BW}$and the right block function 920 of bandwidth F_(BW). As described abovefor a symmetric intensity distribution, the present asymmetric spectralradiant intensity distribution may be converted into a weight functionW(f) proportional to the spectral distribution of radiant intensity I(λ)by expressing I(λ), or in this embodiment by expressing the blockfunctions representing I(λ)) as a function of (λ−λ_(c)), and writingλ−λ_(c) as an equivalent focal coordinate f with (λ−λ_(c))=f/AC, in viewof the lens property df/dλ=AC. Since the block functions 910 and 920 areof equal area, the exposure dose in the corresponding focus ranges isequal.

The effect of a change of the spectral intensity distribution whichinitially is representing a quasi monochromatic laser line into anasymmetric spectral intensity distribution on a Bossung curve can beestimated using the procedure as described above.

A combination of the present approximation for the intensitydistribution I(λ) (resulting in to adjacent block-shaped weightfunctions) with the approximation CD_(iso) (0,2; f) for the printed CDof an isolated feature, results in the following prediction for theaverage critical dimension CD_(av) (at arbitrary defocus F):

$\begin{matrix}{{{CD}_{av}(F)} = {{\frac{1}{2}\frac{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF}}{{{CD}_{iso}\left( {0,{2;f^{\prime}}} \right)}\ {\mathbb{d}f^{\prime}}}}{\int_{F + f_{BF} - {\frac{1}{2}F_{BW}}}^{F + f_{BF}}{\mathbb{d}f^{\prime}}}} +}} \\{\frac{1}{2}\frac{\int_{F + f_{BF}}^{F + f_{BF} + F_{BW}}{{{CD}_{iso}\left( {0,{2;f^{\prime}}} \right)}\ {\mathbb{d}f^{\prime}}}}{\int_{F + f_{BF}}^{F + f_{BF} + F_{BW}}{\mathbb{d}f^{\prime}}}} \\{= {A_{0} + {A_{2}F^{2}} + {\frac{1}{2}A_{2}{FF}_{BW}} + {\frac{1}{4}A_{2}F_{BW}^{2}}}}\end{matrix}$

As schematically indicated in FIG. 10, not only an offset 900 withmagnitude

$\frac{1}{4}A_{2}F_{BW}^{2}$is introduced (similar to the situation whereby an increase of bandwidthof a symmetric spectral distribution is applied) but also a linear term

$\frac{1}{2}A_{2}{FF}_{BW}$is introduced. The presence of these two contributions results in ashifted and counter-clockwise tilted Bossung curve (910), asschematically indicated in FIG. 10. Further, the focus position alongthe optical axis where a change of critical dimension as a function of achange of focal position is zero, is now located at a defocus positionF_(iso) slightly defocused from the best focus position f_(BF).

This tilt could be used to compensate for spherical aberration of theprojection lens, characterized by for example the Z9 Zernike coefficient(resulting in a Bossung tilt of a Bossung curve, which in the absence ofsherical aberration would be untilted). Thus, if the initial Bossungcurve 600 would have been tilted clockwise, the counter-clockwise tiltof curve 910 would then result in the Bossung curve 910 being untilted,i.e., symmetric with respect to best focus. Also this tilting behaviorcould be used to influence side wall angles and pattern collapsebehavior.

Referring to FIG. 11, examples of Bossung curves 140, 141, 142 show theimpact of a transition from a conventional relatively narrow andsymmetrical spectral intensity distribution (143) to a symmetricalbandwidth-broadened distribution (144) and to an asymmetrical spectralintensity distribution (145). The dashed lines indicate theapproximation used for the weight function W(f). The Bossung curve fordense lines is not shown, and is unaffected, thereby providing twoindependent parameters for adjusting an iso dense bias characteristic ofan apparatus. Note for both the symmetrical and asymmetrical case thetotal focal range 146 is the same.

Turning now to FIG. 12 there is shown the simulated effect of symmetricincrease of FWHM (Full Width Halve Maximum) for nominal 65 nm isolatedlines (Prolith 5 pass calculation, NA 0.93 and sigma 0.94/0.74, binaryreticle, calibrated resist model). As expected from the calculations,the constant decrease in CD through focus by introduction of an enlargedlaser bandwidth. Note all calculation are performed using the sameexposure dose.

Referring to FIG. 13 there is shown a symmetric laser bandwidthdistribution as used for the simulations in FIG. 12. For the simulationsthese laser bandwidth distributions were approximated.

The impact of varying the asymmetry of a spectral intensity distributionI(λ) is shown by way of simulations and illustrated in FIG. 14 and FIG.15. FIG. 14 shows different asymmetric spectral intensity distributions111, 112, 113, and 114. For the simulations these spectral intensitydistributions were approximated. FIG. 15 shows the simulated effect ofincreased spectral asymmetry for constant FWHM (Full Width HalfMaximum=0.2 pm), and for nominal 65 nm dense and isolated lines (Prolith5 pass calculation, NA 0.93 and sigma 0.94/0.74, binary reticle,calibrated resist model). The Bossung curves 111′, 112′, 113′, and 114′correspond to the respective spectra 111, 112, 113, and 114. As expectedfrom the calculations, the effect is a shift of the Bossung curve alongthe focus-axis and change of the tilt of the Bossung curve at a fixedfocus. Note that all calculations were performed using the same exposuredose.

Referring finally to FIG. 16, there are shown the same results asdepicted in FIG. 16 after correcting for the focus shift showing that,as expected from the calculations, the Bossung curve has tilted due tothe introduction of an asymmetric laser bandwidth for constant FWHM(Full Width Halve Maximum=0.2 pm) for nominal 65 nm isolated lines(Prolith 5 pass calculation, NA 0.93 and sigma 0.94/0.74, binaryreticle, calibrated resist model). Note all calculations were performedusing the same exposure dose.

Image smear-out due to a symmetric spectral intensity distribution, andapproximated by a block shaped weight function W(f) as illustrated inFIG. 7, is fully analogous to image smear-out due to a contiuousz-movement of the substrate during exposure of a target portion. Thisanalogy is schematically illustrated in FIGS. 17 and 18. Referring toFIG. 17, there is shown a schematic representation of introduction of awafer Rx tilt (a roation around an axis parallel to the x-axis) showingthat when exposing the wafer with Rx tilt, the wafer is exposed atdifferent focus positions (−F to +F) during the scan. In scan directioneach point of the wafer sees a through focus behavior ranging from−a·R_(x) to a·R_(x). (2a is slit width).

The imaging situation as is presented in FIG. 17 can be also achieved byusing a finite laser bandwidth. In general wavelength has an impact onbest focus position, so by stretching the laser bandwidth has a similarresult on the aerial image of a structure as wafer tilt.

Referring to FIG. 18, there is shown a schematic representation ofeffect of wafer-Rx tilt and laser bandwidth stretching on focus and doseseen by the structure to be imaged as compared to normal exposure.

An effect on average CD due to a continuous change of focus position ofthe substrate during exposure can be modeled by the equations 4-8, byreplacing

$\frac{1}{2}F_{BW}$with half the range of focus displacement of the substrate. Thus, todescribe the effect on printed CD in case a substrate is tilted and ascanning movement is executed along the tilted direction, as illustratedin FIG. 17,

$\frac{1}{2}F_{BW}$is to be replaced by a·R_(x) in equations 4 to 8. In view of thisanalogy, and according to an aspect of the present invention, residualtilt and tilt-variations of the substrate, present during exposure, canbe supplemented with a complementary constant as well as variableportion of bandwidth of the spectral intensity distribution I(λ), sothat the added effect of residual tilt and finite spectral bandwidth isconstant during exposure, thereby improving CD uniformity.

According to an aspect of the invention, a control interface which ispart of the controller CON in FIG. 1 may be provided between astep-and-scan lithographic apparatus and an excimer laser which is partof the radiation system of the apparatus, and which provides pulsedradiation. The interface is used by the apparatus to send wavelengthsetpoints to the laser on a pulse to pulse basis. The laser has aninternal module which measures the wavelength per pulse. The differencebetween the actual measured wavelength and the wavelength setpoint isused by the lithographic appararatus as an error-signal in a pulse topulse feedback control loop. This enables the controller to adjust thelaser wavelength during exposures. For instance wavelength profiles canbe used during exposures to compensate focus profiles during theexposure. Thus, the radiation peak wavelength λ_(p) can be varied duringan exposure, resulting in a spectral shift of the bandwidth of thespectral intensity distribution I(λ), and the bandwidth shift can beused as an extra manipulator to optimize the lithographic printingprocess. For example, an effect on printed CD of pressure variations andwafer unflatness (as part of a dynamic Field Curvature correction) canbe compensated in accordance with the present invention.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention. It will also be appreciated that the disclosed embodimentsmay include any of the features herein claimed.

1. A lithographic apparatus comprising: a radiation system configured tocondition a beam of electro-magnetic radiation having a spectraldistribution of radiant intensity; a support structure configured tosupport a patterning device, the patterning device serving to impart thebeam of radiation with a pattern in its cross-section, the patternhaving a feature; a substrate table configured to hold a substrate; aprojection system configured to expose a radiation sensitive portion ofthe substrate to the patterned beam of radiation; and a controllerconfigured and arranged to provide an adjustment of said spectraldistribution of radiant intensity based at least on data relating toexposure at a first focal position and exposure at a second focalposition and representing a corresponding first printed size and secondprinted size of the feature.
 2. A lithographic apparatus according toclaim 1, wherein the spectral distribution of radiant intensitycomprises a spectral intensity peak having a bandwidth and wherein saidadjustment comprises a change of the bandwidth.
 3. A lithographicapparatus according to claim 2, wherein the spectral distribution ofradiant intensity is a superposition of a first and a second peakedspectral intensity distribution having a respective equal first andsecond bandwidth, and a respective equal first and second intensity, anda respective first and second peak wavelength, and wherein theadjustment comprises a change of difference between the first and secondpeak wavelength.
 4. A lithographic apparatus according to claim 1,wherein the spectral distribution of radiant intensity comprises aspectral intensity peak having, with respect to a center wavelength, asymmetric shape and wherein said adjustment comprises a change of thesymmetric shape into an asymmetric shape with respect to the centerwavelength.
 5. A lithographic apparatus according to claim 2, whereinthe data represent a difference between the corresponding first printedsize and second printed size of the feature.
 6. A lithographic apparatusaccording to claim 5, wherein the data further comprise a targetdifference between the corresponding first printed size and secondprinted size of the feature.
 7. A lithographic apparatus according toclaim 6, wherein the adjustment of said spectral distribution of radiantintensity is arranged to match the difference to the target difference.8. A lithographic apparatus according to claim 7, wherein the targetdifference is a difference between the corresponding first printed sizeand second printed size of the feature, as printed using the patterningdevice on a supplementary lithographic apparatus.
 9. A lithographicaccording to claim 1, wherein the radiation controller is configured tocontrol a source of the beam of radiation.
 10. A lithographic apparatusaccording to claim 1, wherein the spectral distribution of radiantintensity is a superposition of a first and a second peaked spectralintensity distribution having a respective first and second bandwidth,first and second peak wavelength, and first and second intensity, andwherein said adjustment comprises a change of one or more selected from:a difference between the first and second peak wavelength and differencebetween the first and second bandwidth, a difference between the firstand second peak wavelength and difference between the first and secondintensity, and/or a difference between the first and second peakwavelength and difference between the first and second bandwidth anddifference between the first and second intensity.
 11. A lithographicapparatus according to claim 10, wherein the difference between thefirst and second peak wavelength is selected from the group consistingof between 0 and 1 pm.
 12. A lithographic apparatus according to claim1, wherein the spectral distribution of radiant intensity comprises aspectral intensity peak having a bandwidth and wherein the radiationsystem comprises an excimer laser to provide the beam of radiation andhaving a bandwidth-controller arranged to control the bandwidth andwherein the bandwidth-controller is constructed and arranged to adjustthe bandwidth, a peak wavelength of the spectral intensity distribution,a shape of the spectral intensity distribution, and/or the bandwidth andthe peak wavelength, in reaction to a user supplied signalrepresentative for a respective selected bandwidth, peak wavelength,shape, and/or bandwidth and peak wavelength of the spectraldistribution.
 13. A lithographic apparatus according to claim 12,wherein the signal representative for a selected bandwidth of thespectral distribution is provided by the controller.
 14. A devicemanufacturing method comprising: providing a beam of electro-magneticradiation having a spectral distribution of radiant intensity;patterning the beam of radiation with a pattern in its cross-sectionusing a patterning device, the pattern having a feature; projecting thepatterned beam of radiation to expose a radiation sensitive portion of asubstrate; and adjusting said spectral distribution of radiant intensityat least in part in accordance with data relating to exposure at a firstfocal position and exposure at a second focal position and representinga corresponding first printed size and second printed size of thefeature.
 15. A method according to claim 14, wherein the spectraldistribution of radiant intensity comprises a spectral intensity peakhaving a bandwidth and wherein said adjusting comprises changing thebandwidth.
 16. A method according to claim 14, wherein the spectraldistribution of radiant intensity comprises a spectral intensity peakshaped symmetrically with respect to a center wavelength and whereinsaid adjusting comprises changing the spectral intensity peak into aspectral intensity peak shaped asymmetrically with respect to the centerwavelength.
 17. A method according to claim 15, wherein the datarepresent a difference between the corresponding first printed size andsecond printed size of the feature.
 18. A method according to claim 17,wherein the data further comprise a target difference between thecorresponding first printed size and second printed size of the feature.19. A method according to claim 18, wherein the adjusting of saidspectral distribution of radiant intensity comprises matching thedifference to the target difference.
 20. A method according to claim 19,wherein the target difference is a difference between the correspondingfirst printed size and second printed size of the feature, as printedusing the patterning device on respectively a first lithographicapparatus and a second lithographic apparatus.
 21. A method according toclaim 14, wherein the adjusting is provided during one of a scanningexposure of a target portion on a substrate and a plurality of scanningexposures of a corresponding plurality of target portions on asubstrate.